The flagellum (Latin, flagellum, whip) is the most conspicuous feature of these motile unicellular creatures for expert and non-expert alike. It it is used to propel the flagellate through the water, and electron microscopy reveals it to be composed of a complex arrangement of protein microtubules. Strictly speaking, the term flagellum now applies only to the quite differently constructed flagellum of certain motile bacteria, but is used in its earlier sense in these galleries. The "flagellum" possessed by protozoans and certain algae is now described as an undulipodium.

Identification of flagellates can be difficult, as most are quite small, requiring the highest powers of the microscope and careful technique to observe their distinguishing features.

Recent work on the structure of mitichondria and analysis of mitochondrial RNA has shown the old class Flagellata contained many similar-looking organisms which were not particularly closely related, and a number of different regroupings based upon this knowledge are now in use. These will be commented upon as the occasion arises.

Peranema. This flagellate is common in waters rich in organic nutrients -- ie, waters in which much decay is taking place. Perhaps counterintuitively, the single flagellum projects straight forward, and a rapid twiddling of its extreme end pulls the Peranema smoothly through the water. Its body can undergo extreme contraction and distortion as it proceeds.
Zonal Illumination: x300.

Another Peranema amongst bacteria amd filamentous algae. Peranema is said to absorb nutrients through its outer pellicle, and it can also ingest quantities of detritus, bacteria, algae and even other organisms of its own size by expansion of the cytostome -- a cavity which lies at the base of the flagellum. It is a continuously active predator and scavenger.
Phase Contrast: x700.

Here a single Peranema has moved into an empty tubule of the filamentous alga Oedogonium from which the algal zoospore has only recently escaped. Perhaps it is absorbing nutrients remaining in the tubule (?).
Brightfield: x800.

A Peranema swimming freely in open water. The extended flagellum is for the most part held straight, with only the end eighth or so moving rapidly and pulling the Peranema smoothly forward in a 3 o'clock direction.
Brightfield: x1400.

The picture shows an air bubble encroaching upon the field at the upper left, and a zone consisting of large numbers of the flagellate Chilomonas formed in a band parallel to the air-water interface. This is a good example of the "chemotaxis" ascribed to most micro organisms -- their ability to respond to levels of nutrients and other dissolved substances in the surrounding water, and to move towards the most favourable concentration.
There is a gradient of dissolved oxygen in the water which decreases with increasing distance from the bubble, and the Chilomonas have located themselves in the zone which is just right for them. Close examination reveals a fainter zone slightly further out consisting entirely of bacteria which clearly favour a similar oxygen level. It is also possible that both the bacteria and the flagellates are producing substances which act as attractants to their fellow organisms.
Darkfield: x50.

Two Chilomonas. Correct photographic exposure for the body does not allow their two flagellae to register on the film (see diagram).
Darkfield: x1000.

Movie: 900KB.
Takes about
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Here is a short video sequence of an active field of Chilomonas in a pondwater sample which had been left to stand for a couple of weeks, and had developed a dense population of bacteria, ciliates and diatoms. Large numbers of vibrio bacteria can be seen in the background.
The behaviour of these bacteria is shown in more detail in this short video sequence (470KB -- takes a minute and a half to load).

The sequence was videoed with an exceedingly cheap digital camera stuck to the eyepiece of a laboratory microscope with Bluetak. The objective was a Russian LOMO x70/1.23NA water immersion apochromat with a narrow bandpass green filter in the illumination path. The automatic white balance of the camera effectively compensated the deep green of the filter to produce a fairly neutral image.

Here is a link to an article which describes the use of the above-mentioned digital camera in videomicrography with an Open University McArthur microscope.
Brightfield: x1500.